Like Amy, I also have the adjustable strap seat on my TR, where when new I had to tighten it every so often, but the last few times we tried it wouldn't tighten anymore. If possible, I would never own standard (non-adjustable) seat upholstery again.

Keep in mind that if you go with a solid seat pan, you will sit higher than with upholstery, so you may need to calculate for that.